Wired LAN and Backbone Requirements
Quality of Service Recommendations
Wired LAN and Backbone Requirements
Quality of Service Recommendations
To be able to provide voice grade communication over WLAN, the use of WMM or 802.11e is
a necessity. These standards define the mapping of priorities on the WLAN to priorities on the
wired LAN using either Layer 2 (CoS, Class of Service) or Layer 3 priorities Differentiated Ser-
vices Code Point (DSCP).
Traffic shaping in the switches should be avoided and instead packet-based priority by the Sta-
tions (STAs) should be used. Each packet is prioritized, according to the standards mentioned
above, depending on the packet type. Priority is primarily needed for wireless prioritization and
secondarily for wired LAN prioritization.
The User Priority (UP) or DSCP value of the frame determines what Access Category (AC) will
handle the frame.
Four ACs are defined in the WMM specification:
• AC_BK (background)
• AC_BE (best effort)
• AC_VI (video)
• AC_VO (voice)
WMM maps the UP used in the 802.11 frames to a corresponding priority on the wired LAN
802.3 frame.
• Layer 2 priority uses the 802.1p priority field in the 802.1Q VLAN tag, on the wired side of the
AP/controller.
• Recommended value for 802.1p priority for voice is 6. For both the wired and wireless side of
the AP or controller.
• Recommended value for the DSCP value is 46 (EF, Expedited Forwarding) for Real-time
Transport Protocol (RTP) frames.
• SIP signalling DSCP value (0x1A (26), Assured Forwarding 31 for both handset types).
IEEE 802.11 Priority Field
The 802.11 UP is sent using the 2-bit QoS Control Field in the 802.11 Medium Access Control
(MAC) header.
IEEE 802.1q Priority Field
The structure of the VLAN tag defined in 802.1Q is illustrated in
on page 10.
8
7
6
5
4
3
2
1
8
7
6
5
4
3
2
1
2
1
Prirority Mark
VLAN Identif er (VID)
= 1 Bit
Octets
Figure 1: Structure of a VLAN Tag
A31003-M2000-P103-01-76A9, 29/04/2020
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System Planning, Planning Guide
